Sea spiders provide insights into Antarctic evolution

Sea spider expert, Dr Claudia Arango, visited the Australian Antarctic Division this week to study a large collection of these long-legged marine invertebrates acquired during recent Antarctic voyages.

Dr Arango, from the Queensland Museum, is working with scientists from the Antarctic Division, the Institute for Marine and Antarctic Studies, and research institutes in Germany, Spain, the UK and the US, to study the diversity and evolution of Antarctic sea spiders (or ‘pycnogonids’). The work will provide insights into the evolutionary history of Antarctica and the movement of species into and out of the continent.

“Pycnogonids are very ancient; we know of a larva from the Cambrian period 500 million years ago and an adult from the Silurian period about 425 million years ago,” Dr Arango says.

“They are also widely distributed and diverse. They occur in tropical, temperate and polar oceans, from shallow water to abyssal depths; they range in size from 1 mm to over 70 cm in leg span; and about 20% of the world's known species – about 1350 – are found in Antarctica.”

The age, distribution and diversity of sea spiders makes them useful for studying evolutionary history because large-scale events, such as glaciations and the isolation of Antarctica by the Antarctic Circumpolar Current, can be tracked by trends in the distribution of pycnogonid species (as well as other invertebrates such as octopus and isopods). To detect such trends Dr Arango is initially examining the distribution pattern of the most common pycnogonid species, Nymphon australe.

A key part of the work involves DNA analysis, which will allow Dr Arango to separate morphologically similar species (looking alike), which may have been mistaken for the same species, and to look at how related different populations of a species are in Antarctica and globally.

Another question Dr Arango hopes to resolve is how sea spiders disperse. Most marine invertebrates – take corals or sea urchins for example – scatter eggs, sperm or larvae into the ocean currents, where they're distributed far and wide. Sea spider eggs and larvae tend to remain attached to the adult male. One option for greater dispersal is via a ‘host’ animal in which the sea spider hides, which may be dispersed on ocean currents, such as hydroids (plant-like animals related to jellyfish) or rafting algae. Genetic analysis of different sea spider populations and species should provide some insights into how or if they are dispersing.

While at the Antarctic Division, Dr Arango was busy identifying sea spiders collected during sea floor trawls in the Davis region last year (see Australian Antarctic Magazine 18: 1–3, 2010). She sorted them visually using a microscope and took small samples for DNA analysis. So far she has identified 27 species. She has also recently identified and sorted about 60 species from some 500 samples collected during the Collaborative East Antarctic Marine Census in 2007.

“The diversity in Antarctica is amazing and the number of individuals and species collected in samples from Antarctica is higher than anywhere else in the world,” she says.

“There also seems to be a trend towards larger sea spiders as you go deeper, but there are small, slender ones in Antarctica as well.”

Dr Arango says not much is known about where sea spiders fit in the sea floor ecosystem, other than that they are carnivores that feast on the juices of hydroids, bryozoans and possibly ascidians and sponges, using a long, slender proboscis.

“Sea spiders don't usually turn up in the gut of fish, so we don’t know if they’re preyed upon. It is probably accidental predation, when the animals they're sheltering in are eaten,” she says.

Dr Arango is obviously passionate about these unusual creatures, which look a lot like terrestrial spiders. But despite her familiarity with the spidery genre, she admits to a sinking feeling when her colleague, an arachnid expert, asks her to look after his live tarantulas.